In the technology of building books including booklets, magazines, periodicals, and so on, the use of collating systems is well known. These systems typically have a transporting device on which individual signatures are gathered to build a book-block set, which then is finished and bound. Typically a number of hoppers is arranged along the transporting device where each of the hoppers comprises a feeding mechanism for feeding an individual signature from a pile of signatures onto the transporting device, in order to progressively build up the book-block set. Such feeding mechanisms typically employ callipering devices for detecting a malfunction of any of the feeding mechanisms on a given line. Such a malfunction may, for example, result in a delivery of more or less than the intended number of sheets onto the transporting device.
To alleviate these problems, some efforts have previously been made to design collating systems having callipering devices for measuring the thickness of sheets being delivered from a pile of signatures to a transporting device. These calipers typically employ a mechanical feeder gauge which is calibrated to measure the thickness of the sheets being fed from a hopper to a transporting device. Such a thickness measurement device is for example known from U.S. Pat. No. 4,471,954, the disclosure of which is hereby incorporated by reference. This document discloses a transporting device in the form of a pocket feeder having a plurality of sheet loading pockets. Each of the pockets is associated with a sheet ejector for ejecting single sheets into a common sheet conveyor path from the respective pocket. A photo-electric sheet passage detector is mounted between each pocket and the common sheet conveyor path which generates a transparency or reflectivity signal of the sheet and which is compared to a stored averaged signal representative for a single sheet.
Another apparatus for measuring the thickness of sheets being fed from a hopper to a pocket feeder is known from U.S. Pat. No. 5,213,318, the disclosure of which is hereby incorporated by reference. The apparatus of U.S. Pat. No. 5,213,318 comprises a plurality of hoppers for holding signatures where the plurality of pockets is movable past the plurality of hoppers. Each of the pockets has a first fixed roller and a second movable roller being in contact with each other when no sheet is fed between the two rollers. When a flat product passes between the two rollers the second roller is deflected, where the amount of the deflection corresponds to the thickness of the product passing. In order to measure the thickness, the deflection of the second roller is measured. The second roller therefore is mounted on a movable shaft which comprises a plate on one of its ends. The plate is rotatable and arranged within a gap between a light source and a light sensor. Depending on the degree of rotation of the shaft the plate has a corresponding angular position and depending on this angular position it interrupts more or less light from the light source where the amount of interrupted light corresponds to the deflection the rollers.
The accuracy of this method however is limited due to the shadow-effects and due to the limited resolution of the light sensor. Furthermore, every pocket has to be provided with a separate light source.